https://github.com/linkerd/linkerd2/pull/2521 introduces an "Identity"
controller, but there is no way to include it in linkerd installation.
This change alters the `install` flow as follows:
- An Identity service is _always_ installed;
- Issuer credentials may be specified via the CLI;
- If no Issuer credentials are provided, they are generated each time `install` is called.
- Proxies are NOT configured to use the identity service.
- It's possible to override the credential generation logic---especially
for tests---via install options that can be configured via the CLI.
Oliver Gould